In curtain coating, a traveling web or object is coated by a free-falling curtain of coating liquid that is caused to impinge onto the traveling web or object to form a layer thereon. The width of the free-falling curtain is maintained by edge guides that are in wetting contact with the lateral boundaries or edges of the curtain. For example, in aqueous solution systems for photographic products it is known to use a curtain coating method which comprises the simultaneous application of silver halide emulsions containing gelatin as a binder, by using a slide hopper coating apparatus to form a multilayer photographic film or paper. A moving web is coated by a free-falling curtain of coating composition wherein the multilayer composition is formed on the slide hopper and caused to fall as a curtain and impinge the moving web to form the coated layer thereon. The formation of a composite of a plurality of distinct layers on a moving web is described in U.S. Pat. No. 3,508,947, which relates particularly to the manufacture of multilayer photographic materials such as photographic film and paper.
In the process of curtain coating, the coating hopper is usually filled and flushed with water or cleaning solution for some time prior to being filled with coating solutions. This procedure will henceforth be referred to as hopper preparation. As the hopper is flushed with water and the water is purged from the hopper by the coating solutions, the flow through the hopper may not be steady. In other words, during hopper preparation it is con, non to encounter flow rate pulsations on the hopper slide and in the curtain. During this time it is difficult to maintain a full width curtain without breaking due to the unsteady flow and fluid inhomogenities. This generally causes the curtain to degenerate and break into an unsteady, uncontrolled flow. As these solutions leave the hopper lip they have a long distance to fall before they contact a solid surface. The solutions accelerate as they fall. The result of the impingement of the high velocity unsteady fluids on a solid surface is splashing in the coating area. This splashing or splattering can contaminate equipment with water and/or coating solutions.
Contamination of equipment such as the boundary air vacuum baffle, edge guides or high speed start/finish pan can result in unacceptable coating quality causing waste in manufacturing. During hopper preparation, the hopper is not directly over the coating roller or the coating impingement point. The hopper is translated to a position in front of the coating roller in an effort to keep the coating point free of contamination. However, even though some distance separates the curtain from the coating point, splashing may still contaminate the surfaces unless the fluids are well controlled in the hopper preparation position.
It is not practical given the economics, design optimization and space constraints to locate the hopper preparation position far enough from the coating position to prevent contamination at the coating point. For this reason, a device known as a preparation pan is inserted directly below the hopper lip during hopper preparation. The preparation pan intercepts most, but not all, of the fluids flowing off the hopper lip during hopper preparation. Since the preparation pan cannot physically interfere with the edge guide, its width is less than the curtain width and it cannot intercept the fluids that flow off the hopper lip and fall in the vicinity of the edge guides. As a result, while the preparation pan is fairly successful in preventing contamination of the equipment located near the coating point, it does not prevent contamination of the edge guides. Even if equipment in the area of the coating point is kept free from contamination, contamination of the edge guides is a significant problem. The splashing in the hopper preparation position frequently contaminates the edge guides. Any disturbance to the natural path desired by the curtain will result in a wave in the curtain. A stationary disturbance, such as contamination on the edge guides, will result in a standing wave in the curtain. A standing wave will result in a longitudinal streak being seen at the position on the support that the wave strikes which can be several centimeters in from the edge guide. In addition to causing a streak at the point at which they strike the support, waves can also redistribute fluid in the entire area underneath them, therefore compromising the uniformity of the coating from the edge to the wave.
The present invention is an apparatus which improves the preparation of a coating hopper by reducing the amount of splashed material which can cause contamination of the surrounding coating equipment.